The present invention concerns a method and device for controlling the recirculation of exhaust gas in a pressure wave supercharger for an internal combustion engine.
It is usual in internal combustion engines, in order to reduce the emission of pollutants, particularly the proportion of oxides of nitrogen and hydrocarbons, to mix exhaust gas with the combustion air in internal combustion engines, into the induction air in the case of naturally induced engines and into the compressed boost air in the case of supercharged engines.
With this return or recirculation of a part of the exhaust gases into the combustion air, the problem of the most effective metering of the returning exhaust gas proportion as a function of the engine load arises with all types of superchargers.
A primary recirculation of exhaust gas into the compressed boost air, i.e. one functioning without additional devices, by mixing air and exhaust gas at the separating line of the two media in the pressure wave supercharger can be obtained in a certain speed and load range of the engine, for example in the most heavily used range of a driving cycle, by an arrangement of the control edges, in which the expanded exhaust gas is not completely scavenged in the low pressure part. The proportion of the exhaust gas which is not scavenged is compressed in the high pressure part along with the fresh air and goes with the latter into the engine. The effectiveness of the high pressure side and the idle running properties of the engine are both worsened by this primary recirculation control by means of the control edge geometry. Since this recirculation control occurs by means of the low pressure scavenging, it is also affected by the low pressure resistances, which can change in the course of time, due particularly to filter dirtying.
Various solutions have been proposed for improving the recirculation variation over the whole operating range of pressure wave superchargers, for example by the Swiss Pat. No. 552,135. This patent describes a method and a device in which, in addition to the primary exhaust gas return, a secondary exhaust gas return is provided for the purpose of controlling the recirculation over the whole load range. According to FIGS. 7 and 8 of this Swiss patent, exhaust gas is branched off from the exhaust gas high pressure duct 5 and, at one position at least at which the cells of the rotor are filled with air, is introduced directly into the pressure wave process. The elements required for this purpose, however, complicate the supercharger and produce difficulties on installation in the vehicle. From the thermodynamic point of view, this solution has the disadvantage that the difference between the boost air pressure and the high pressure exhaust gas and also the boost pressure are reduced because of the extraction of high pressure exhaust gas. It is, however possible in this manner to obtain a better, matching of the degree of recirculation to the operating condition of the engine at any time than is possible by a primary exhaust gas return alone.
An objective of the present invention is to reduce the disadvantages of the known solutions outlined above with the smallest possible expenditure of constructional effort. In particular, the following requirements should be fulfilled:
The increase of the recirculation may only slightly worsen the efficiency of the pressure wave process on the high pressure side,
it should be possible to optimize the high pressure side pressure difference and the level of the boost pressure independently of the recirculation criteria,
it should be possible to optimize the degree of recirculation independently of the requirements of idling operation, and
the degree of recirculation shall remain constant independently of the induction resistance, due for example to dirtying of the filter.
These objectives of the invention are attained by controlling the low pressure side scavenging alone, but the control edge geometry most favourable for the best pssible supercharging can be retained.